drm/msm/dsi: Pull out panel init code into function

Pull all of the panel init code out of detect() and put it in its own
function. This will be useful in future patches where it's moved from
detect().

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-6-sean@poorly.run